Chapter 5 Volcanoes

FIB: An blank blank has formed in the crater of Mount St. Helens.

Andesitic dome

Important Concept # 6

A volcanic eruption can have a global impact on climate and atmospheric chemistry through the production of volcanic dust and/or sulfur-rich gases.

hot spots

an area that is very hot and rises on the border of the mantle outer core and the pacific plate.

MC:Mafic (iron-rich) lavas

are low in viscosity and flow easily.

MC: Most volcanic activity on land areas occurs

in association with subduction zones.

MC:The catastrophe at Lake Nyos, Cameroon, Africa, in 1986 was caused by

the release of carbon dioxide.

Cinder Cone

small-volume, generally mafic volcanoes composed of pumice / scoria. Is monogenetic.

MC:Volcanoes of the Cascade Range are

stratovolcanoes.

MC:Volcanoes composed of pyroclastics are called

Cinder cones

Review Question 2

Discuss the distinctions among active, dormant, and extinct volcanoes. Comment on the limitations of this classification scheme.

Review Question 6

Do you expect the Yellowstone National Park region to be a potential candidate for future volcanic activity? Justify your answer.

FIBA volcano considered to be blank is very unlikely to erupt again.

Extinct

T/F:Lava flows are among the most hazardous of volcanic activities because they are the least predictable of volcanic hazards.

False. Lava flows can be hazardous; however, they are at least predictable, since their flow paths can be anticipated.

T/F: The major eruption of Mount Pinatubo in 1991 came as a complete surprise to volcanologists and local residents.

False. Mount Pinatubo exhibited several volcanic precursors prior to its main eruption on 15 June 1991; these included increased seismic activity, a bulge on the volcano's summit, emissions of ash, and a sudden drop in gas output.

T/F: There are an estimated 3000 to 5000 active volcanoes in the world.

False. There are an estimated 300 to 500 active volcanoes in the world.

MC: Which of the following is not a type of pyroclastic material?

Flowing Lava

Important Concept # 7

In terms of their activity, volcanoes can be classified as active, dormant, or extinct.

MC:A well-known volcano associated with a rift zone is

Kilimanjaro.

Review Question 1

List the different products (direct or indirect) of volcanic eruptions and the hazards they may cause. What, if anything, can be done to minimize or eliminate these hazards?

mafic

Magma or rock relatively rich in iron and magnesium. -Low Silica -Low Viscosity -High Temperature -Low Gas Content -Usually Produces Lava Flows

lava

Molten rock exposed on the earth's surface.

Important Concept # 1

Most magmas originate in the upper mantle, at depths between 50 and 250 kilometers, and are typically generated (1) at divergent plate boundaries, (2) over subduction zones, and (3) at hot spots.

Important Concept # 2

Most volcanoes are located over subduction zones, although a few volcanoes are associated with mantle hot spots. Fissure eruption, rather than eruption from a single pipe or vent, accounts for the immense volume of volcanic rocks formed at the divergent boundaries (spreading centers). The very extensive basaltic rocks of the Columbia Plateau in the western United States and of the Deccan Plateau in western India were formed by fissure eruption on continents.

FIB: blank is the volcanic, fine-grained compositional equivalent of granite.

Rhyolite

FIB:The collection of volcanoes rimming the Pacific Ocean is known as the blank.

Ring of fire

MC: The Hawaiian peak called Mauna Loa is an example of a

Shield Volcano

T/F:Silica-rich magmas are less viscous, thinner, and flow more easily than silica-poor magmas.

Silica- rich magmas are more viscous, thicker, and flow more sluggishly than silica-poor magmas.

Important Concept # 3

The chemical composition of a magma influences its physical properties. Silica-poor, iron- and magnesium-rich magmas (mafic magmas) are low in viscosity and flow easily. Silica-rich magmas (felsic magmas) are viscous and flow sluggishly. Felsic magmas tend to trap dissolved gases, which may lead to explosive eruptions. Explosive eruptions are potentially more hazardous than nonexplosive (passive) eruptions.

Important Concept # 8

The likely regions of future volcanic activity can be predicted from the present-day plate tectonic map, but not the precise location or time of eruption. Selected volcanoes are routinely monitored for advance warning of volcanic activity. The volcanic precursors geologists look for are seismic activity, bulging, uplifting, or tilting of a volcano's surface, thermal changes, and changes in the mix of released gases.

Important Concept # 5

The primary source of hazards related to volcanic eruptions are the products of eruptions: lava flows, pyroclastic flows, lahars (volcanic mudflows), ash falls, toxic gases (mainly SO2), and phreatic eruptions.

Important Concept # 4

The two main products of volcanic eruptions are lavas and pyroclastics (fragmental volcanic material). Landforms produced by volcanic eruptions reflect the kind of material erupted: (a) A shield volcano is a low, flat, gently sloping volcano built from many flows of fluid, low-viscosity lava. The Hawaiian Islands are composed of shield volcanoes. (b) A volcanic dome is a compact, steep-sided structure built of very viscous, rhyolitic and andesitic lavas emitted from a central pipe or vent. A volcanic dome has formed in the crater of Mount St. Helens left by its 1980 explosive eruption (although the Mount St. Helens volcano as a whole is a composite volcano). (c) A cinder cone is a symmetric, cone-shaped volcano formed from the accumulation of pyroclastics close to the volcanic vent. (d) A composite volcano (or stratovolcano) is built of interlayered lava flows and pyroclastics. They tend to have viscous, gas-charged lavas and can erupt explosively. Most of the volcanoes in the Cascade Range of the western United States are of this type.

T/F: Most volcanoes are located over subduction zones.

True

T/F: The violent eruption of the Indonesian island of Krakatoa in 1883 was an example of a phreatic eruption.

True

T/F: It is possible that animals might "foresee" volcanic events and that studying their behavior might provide volcanologists with clues to impending eruptions.

True

T/F: The eruption of Mount St. Helens in 1980 made many Americans realize for the first time that they lived in areas threatened by volcanoes.

True

T/F: Volcanic eruptions that spew sulfur-rich gases into the atmosphere may enhance the effects of acid rain.

True

T/F:The most energetic pyroclastic eruptions are more typical of volcanoes with viscous lavas ,because thicker lavas tend to trap more gases.

True

Review Question 7

What is a fissure eruption? Give examples of active volcanism and ancient volcanism through fissure eruption.

Review Question 4

Why is there a general coincidence between the distribution of documented earthquake epicenters and young volcanoes? Discuss the exceptions to this general distribution pattern.

FIB: A blank is an enlarged volcanic summit crater formed by either an explosion or the collapse of a volcano.

caldera

MC: Volcanologists

can anticipate the likelihood of an explosive eruption.

pyroclastic flow

dense clouds of ash, lava, and hot gases that form from a collapsing eruption column or from the collapsing of a lava dome

lahar

dense fast moving flows of water, ash, mud and rock.

phreatic eruption

explosions that occur when ground water or meteoric water is flashed to steam by magma in the subsurface. No fresh material.

MC:The Columbia Plateau is an example of a

fissure eruption

MC:A pyroclastic outburst of a denser-than-air mixture of hot gases and fine ash is known as a

pyroclastic flow

FIB:The town of St. Pierre, on the Caribbean island of Martinique, was destroyed by a blank blank that occurred during the eruption of Mont Pelée in 1902.

pyroclastic flow (or nuées ardentes)

FIB:Bits of violently erupted volcanic material are collectively called blank

pyroclastics

ultramafic

relating to or denoting igneous rocks composed chiefly of mafic minerals.

pyroclastics

relating to, consisting of, or denoting fragments of rock erupted by a volcano.

Felsic

relatively silica-rich magmas. -High Silica -High Viscosity -Lower Temperature -High Gas Content -Explosive Eruptions

fissure eruption

relatively silica-rich magmas. -High Silica -High Viscosity -Lower Temperature -High Gas Content -Explosive Eruptions

MCMost volcanic rock is created at

seafloor-spreading ridges

FIB: During the A.D. 79 eruption of Mount Vesuvius, many people were killed by the release of blank blank

toxic (or volcanic) gases

MC: Concerning responses to eruption predictions, the safest course is to

undertake precautionary evacuations.

MC: Most magmas originate in the

upper mantle

Basalt

volcanic rock that display a column structure

MC: Subduction is the underlying cause of the

volcanoes of the Cascade Range of the Pacific Northwest.

stratovolcano

volcanoes that erupt somewhat different materials at different times: layered.

Important Concept # 9

Areas in the United States vulnerable to present and future volcanic hazards include the Aleutian Islands of Alaska, Hawaii, the Cascade Range, the Mammoth Lakes/Long Valley area of California, and the Yellowstone National Park

Review Question 5

Name three areas of active volcanism in the United States (including Alaska and Hawaii), and discuss the plate tectonic setting of each.

Active Volcano

has erupted within recent history.

Review Question 3

What are the precursor phenomena that can be used to predict a volcanic eruption in an area being monitored for such indications? How successful were the geologists in predicting the Mount St. Helens eruption of 1980?

Andesite

a dark, fine-grained, brown or grayish volcanic rock that is intermediate in composition between rhyolite and basalt.

rhyolite

a pale fine-grained volcanic rock of granitic composition, typically porphyritic in texture.

Composite volcano

a tall, cone-shaped volcano

Dormant volcano

a volcano that is not currently active, but is able to become active in the future

MC:All of the following are possible volcanic precursors except

changes in barometric pressure.

FIB:Stratovolcanoes are also called blank volcanoes

composites

MC:Volcanic eruptions may, at least temporarily,

contribute to the formation of acid rain. decrease average global temperature. increase the rate of ozone depletion.

MC:The residents of the Icelandic island of Heimaey stopped a lava flow by

cooling the lava with water.

volcanic dome

is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. The geochemistry of lava domes can vary from basalt to rhyolite although most preserved domes tend to have high silica content.

FIB: A mudflow of meltwater and volcanic ash produced by a volcanic eruption is called a blank

lahar

shield volcano

large, high volume: commonly sub-marine. High flows of basaltic lava. Is polygenetic.

Ash

small particles released from a volcanic eruption.